\input pstfig
$
\psset{unit=1.5em}
\psset{arrowsize=0.3}
\def\Pofs{1.5}
\pnode(4,0){O}
\pnode(8,0){O'}
\pnode(4,\Pofs){P}
\pnode(8,\Pofs){P'}
\pnode(8,-\Pofs){P''}
\psline(4,2)(4,-2)
\psline(8,2)(8,-2)
\thinline
\psline[ArrowInside=->](0,0)(O)
\psline[ArrowInside=->](0,\Pofs)(P)
\psline[ArrowInside=->](P)(P')
\psline[ArrowInside=->](O)(P')
\psline[ArrowInside=->](O)(P'')
\psline(O)(O')
\nLput[offset=-1ex](O)(O'){0.6}{L}
\uput[-135](O){O}
\uput[-135](P){P}
\uput[-135](O'){O'}
\uput[45](P'){P'}
\psset{arrowsize=0.2}
\pcline[offset=1.5em]{<->}(O)(P)
\psset{framesep=1pt}
\mput*{d}
\pcline[offset=1.5ex]{|<->|}(P')(P'')
\mput*{x}
\markang[border=1pt]{<->}(P'')(O)(P'){1}
\uput*{1.15}[0](O){\theta}
% now draw perpendicular line
\psPerpen(P)(O)(P'){Q}
\psline[linestyle=dashed](P)(Q)
\markangrt(P')(Q)(P){0.2}
\uput{0.8ex}[65](Q){Q}
$
\bye
